Document `GIT_STRATEGY` and `GIT_DEPTH`

## What does this MR do?

Documents `GIT_STRATEGY` and `GIT_DEPTH`. Also fixes the TOC since the anchors weren't working.

## Are there points in the code the reviewer needs to double check?

The TOC and the location of the new entries.

## Why was this MR needed?

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/188

## Screenshots (if relevant)

![image](/uploads/eccfd60199d81e6588c13aab817f0eb5/image.png)

![image](/uploads/bbbffe61db64ef17f1fa52907275f3d4/image.png)

/cc @axil @ayufan

See merge request !4720
Signed-off-by: Rémy Coutable <remy@rymai.me>